Process for purification of produced water

ABSTRACT

We provide a process for treatment of produced water, including but not limited to water produced by a “steam flood” process for extraction of oil from oil sands, including the removal of color from the water. This removal may be accomplished through addition of color-removal polymers and flocculents. This process may also be useful for other water treatment processes including reverse osmosis and filtration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/611,806, filed on Mar. 16, 2012, and to U.S. Provisional PatentApplication No. 61/699,524, filed on Sep. 11, 2012. Both of thoseapplications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the invention relate to processes for purification ofwater.

Background of the Related Art

Vertical tube falling evaporators are conventionally used for recoveryof produced water generated in steam flood processes used for heavy oilextraction. The produced water is purified by distillation and used forboilers for generating steam. The steam is injected in the undergroundwell to recover oil, which comes out mixed with produced water. Oil isseparated from water. During oil extraction the produced water picks upsignificant amount of dissolved solids including hardness and silica,and dissolved color and organics. This is usually purified bydistillation after de-aeration and then taken for steam productionagain. During the distillation a goal is usually for more than 95% waterto be recovered. The balance is discharged in salts caverns or injectedin deep wells.

This way overall consumption of water is optimized with minimum make uplosses. This process of producing oil is called the “Steam Flood” or“Steam Assisted Gravity Drainage” Process (SAGD) or “Cyclic SteamStimulation” (CSS). The water treatment is a critical part of thisoperation. The water treatment process requires careful design toprevent or at least minimize scaling and solid build up, to increase uptime of operation, and to improve reliability.

In existing evaporative water treatment processes for steam flood, wateris pretreated to remove oil and subsequently the pH is raised toprecipitate part of the hardness and keep silica in dissolved form. Thepart of precipitated hardness remains in the settlement tank and thereis no carry-over of sludge in the evaporation system. In such a processthere are no solids in the evaporation mixture and the distributionsystem. This water is distilled to recover almost 95-97% water throughvertical falling film evaporators, and the residual brine is neutralizedto reduce the pH as per environmental regulations and discharged in saltcaverns or through deep well injections.

In another evaporative water treatment process, after oil removal silicais precipitated with addition of magnesium oxide and sodium hydroxide atalkaline pH, where silica is adsorbed on the surface of magnesiumhydroxide. In this process resultant precipitated magnesium and silicasludge does not leave the system when water is taken for evaporation.Sludge, including color and organics as a part of water, is continuouslyre-circulated through the evaporator through vertical tube falling filmevaporators. This risks settlement of part of the sludge in thedistribution system on a continuous basis. Over a period of time thismay result in formation of deposits on the surface of tubes and scalingof the tubes.

After the distillation and recovery of 97% water the brine is taken forneutralization. During the neutralization, most of the silica exists inprecipitated form, which may reduce chances of gel formation butrequires that all of the solids be disposed of with brine, which makethe process very expensive and consumes a lot of capacity of saltcavern. Brine generated through this process is not suitable for deepwell injection without extensive treatment for solids removal at thisstage, where solids have sluggish settling and filtrationcharacteristics.

Unfortunately these processes have proven unsatisfactory for a number ofreasons. These include scaling of evaporator surfaces and creation of amore chemically-intensive waste product for disposal to undergroundwells or other areas.

BRIEF SUMMARY OF THE INVENTION

We offer an elegant solution that may simplify operations, reducepossibility of down time, increase reliability and also reduce theoperating cost and disposal challenges of brine. Of course, theinvention is defined by the claims, and not by success or failure in anyone of the above criteria unless included in the claims.

We provide a process suitable for the purification of produced waterfrom steam flood processes. Of course, while the process that we provideis suitable for this use, it is not necessarily limited to it, and thoseof skill in the art will recognize that the general teachings andspecific embodiments may be put to use in other areas where colorremoval would be beneficial.

Processes described herein may have, but are not required to have, oneor more of the following advantages over conventional processes:

-   -   1. A majority of color contributing organics, around 85-90%, are        removed from the produced water in the first step of the        process, which helps in the reduction of 60-70% of TOC (total        organic content).    -   2. Around 95% of silica removal from the produced water helps in        prevent severe scaling in downstream equipment.    -   3. Evaporator foaming is eliminated and scaling is reduced to an        insignificant level, as a majority of dark, tar-like color        chemicals, hardness and silica are removed prior to evaporation.    -   4. Easy neutralization of brine water, no jelly formation.    -   5. Easy sludge/solids disposal.    -   6. Lower chemicals cost & lower maintenance of evaporators &        crystallizers.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow diagram illustrating an overview of a process accordingto one embodiment of the invention for treatment of produced water.

FIG. 2 is a flow diagram according to another embodiment of theinvention, showing a color removal process and further treatment ofwater through an evaporator.

FIG. 3 is a flow diagram according to another embodiment of theinvention, showing color and silica removal processes and furthertreatment of water through an evaporator.

FIG. 4 is a flow diagram of a conventional process for produced watertreatment as described in comparative example 1.

FIG. 5 shows flow diagram of conventional process of produced watertreatment as described in the comparative example 2.

DETAILED DESCRIPTION OF THE INVENTION

Unfortunately, existing processes fail to consider removal of color andorganics, and the need to do so has not heretofore been recognized.Color in wastewater includes compounds with electron-dense functionalgroups that adsorb light in the visible spectrum. Color may also includeorganic products that are the degradation products of decaying wood orother soil organic matter. These products may have colloidal properties,as described in “Coagulation and Flocculation in Water and WastewaterTreatment,” by John Bratby (2^(nd) Ed. 2008), which is incorporated byreference herein and which also describes methods for measurement ofcolor. They may include, for example, fulvic, hymatomelanic, and humicacids. These continue to pose foaming problems during evaporation. Theyalso make it difficult to go to a zero liquid discharge (“ZLD”) stagebecause of the concentration of color and organic contaminants and theresidual ZLD solids make the ZLD product extremely tarry and difficultto handle for any subsequent processing.

In a conventional process of neutralization of brine waste, formation ofa gelatinous substance takes place due to high concentration of residualsilica and precipitation of silica in jelly form. This creates asubstantial disadvantage for discharging waste into salt caverns or deepwell injections and makes such discharge expensive. Presence of colorand organics in sludge may also adversely impact uniform distribution offeed water across several tubes and may also call for frequentmechanical cleaning of the evaporative system.

We report a novel process where disadvantages of existing conventionalwater treatment processes may be overcome or ameliorated. Althoughuseful for steam-assisted gravity drainage (SAGD), this process may alsobe suitable for other processes, including for example pretreatment offeed water for a reverse osmosis process. After removing color and/orsilica the water may be subject to further treatment to remove hardnessbefore it is taken to a reverse osmosis unit.

Embodiments include preconditioning of water. This removes a majority ofcolor forming compounds including organics. It also removes potentialscale forming contaminants with different treatment processes before thewater is sent for further treatment, which may include treatment by anevaporation or membrane unit, as well as solids removal.

A brief overview of an embodiment of the process may be had by recourseto FIG. 1. As shown in FIG. 1, produced water 102 is collected from anoil well 101 and then sent to an oil-water separator 103. Oil is removedand recovered. The water stream 104 then proceeds for removal of colorand organics in color/organics removal tank 105. This is done byaddition of an acid 106 followed by addition of chemical for colorremoval 107 and a flocculent 108.

The stream then proceeds to a solids separator 109. Portions of thestream are immediately disposed as solids after sludge conditioning 110.If desired, the clarified or filtered stream 129 may then be sentdirectly to an evaporator 111 after adjustment of pH to 9.5-10 or above.

More typically the filtered stream 129 flows from the solids separator109 to a mixer 112, where they are mixed with lime 113, magnesium oxide114, and sodium hydroxide 115. The stream 130, then proceeds to a secondsolids separator 115, where a portion of the stream 131 is removed forsludge conditioning 116 and disposal. The reminder of the stream 132proceeds to a de-aerator 117, then to an evaporator 111. The purifieddistillate stream 118 from the evaporator 111 proceeds to a boiler 119,where it is used to make steam 120 for insertion into an oil well. Atthis stage the brine 121 can be neutralized 122 and disposed or brinewith sludge is absorbed in super absorbents and disposed of. In onembodiment the super absorbent is Aquadux® 2212.

As an option, the portion of the evaporator 111 stream that is not sentfor use in a boiler 119, the brine 121, may be sent to a crystallizer124 for removal of additional distillate 125. After this optionalcrystallizer step the slurry 126 is sent directly for disposal or issent for optional sludge conditioning and absorption 127, followed bydisposal.

Embodiments of the invention may now be explored in more detail. In aninitial step of an embodiment of the invention, pH of feed water 104 isadjusted to an acidic range of pH. Preferably the pH is about 3 to 5, orabout 4 to 4.5. This is accomplished with the addition of an acid 106.Acid could be, for example, either hydrochloric acid or sulfuric acid ofsuitable known concentration. Acid addition is followed by addition of acolor removing chemical 107 and a flocculent 108. Majority of the coloris precipitated and removed by filtration.

Typically this process reduces color by around 85-90% and results in aTOC reduction of 60-70%. The color removal process also gives a silicareduction of 15-30%. A color removal process experiment's results arestated in Table 1 of Example-1.

Suitable color removing chemicals include but are not limited to highmolecular weight polymer coagulants, including the polymer Wex-floc-9(from WexTech) or similar color removal chemicals from other suppliers.In one embodiment the compound is a polyamine as reported, for example,in Yue, et al., (School of Environmental Science and Engineering,Shandong University, China), “Synthesis of Polyamine Flocculants andTheir Potential Use in Treating Dye Wastewater.” The dosing of colorremoval chemical can be modified depending on the concentration of colorand organics in the water to be treated. This concentration is oftenaround 25-500 ppm, but typically around 50-100 ppm. The flocculent usedin preferred embodiments is AT 7594 or AT 7595 (WexTech). The dosing isadjusted based on a jar test but it can vary between 0.25-5.0 ppm andmostly between 0.5-1.0 ppm.

After removal of color and associated organics and filtration of thewater, magnesium oxide or dolomite-like compounds containing magnesiumoxide are added under constant agitation in mixer 112. By addition ofmagnesium oxide at this pH, performance of the process improves over aconventional process such that silica reduction improves to less than 10ppm in the filtrate. After this, pH is increased with predeterminedquantities of lime and sodium hydroxide to adjust the pH to about10-11.5, preferably about 11. Due to prior removal of color and silicathe consumption of caustic soda at this step is reduced significantlyfrom the amount necessary in the prior art. The amount of caustic sodamay be reduced by at least 25-30% as compared to the conventionalprocess.

The pH adjustment is followed by addition of a coagulant andpolyelectrolyte. This results in formation of precipitate in the form ofheavy flocks with very good settling properties. The coagulant used atthis stage was AT 7260 (WexTech) or equivalent or similar chemicalssupplied by other suppliers and dosing rate is adjusted based on a jartest. This dosing rate can vary between 1-5 ppm but normally between 2-3ppm.

After providing some settling time in solid separator 115, thesupernatant water 132 can be removed through decanting, centrifuging orany other filtration process. This sludge has very good settlingproperties and usually settles easily within 5-15 minutes. This sludgecan be dewatered and sent for disposal after further sludge conditioning116 and compaction as required based on local regulations. In thisprocess most of the silica and calcium and magnesium hardness areremoved, and water is pre-conditioned to go to the evaporation processwithout solids. The residual silica in water is reduced to around 5-10ppm from 240 ppm, and the total hardness is also reduced to about 20ppm.

The water after color removal process reduces silica in the subsequentprocessing, reduces foaming during evaporation, accelerates the silicaprecipitation during silica removal, and provides easy to handle brineafter the brine concentrator and crystallizer treatments. This is highlyunlike the highly viscous and tarry liquid that is typically produced ifthere is no color removal process to remove organics.

The supernatant liquid 132 is then taken for evaporation in evaporator111. In one embodiment evaporation is conducted in a vertical fallingfilm evaporator. In a preferred embodiment 95-97% water 118 is distilledout and recovered. During distillation the water remains clean with verylittle precipitates, and there is no or effectively no build up orsettlement of solids on the distribution plates and no or effectively nodeposition of solids on tubes. Typically there is no scaling of any kindbecause the majority of the total hardness is removed along with silicaas a part of precipitated solids.

The precipitated solids in brine are separated after settling and takenfor disposal after dewatering with the sludge, which was separated alongwith silica and magnesium precipitates in the first step.

The water 129 after the color/silica reduction step can be directlytaken for evaporation without filtration of solids, if the solidsconcentration is not high after silica reduction and precipitationbecause of additional silica reduction that has happened along withcolor removal process. This is normally done if the silica concentrationin water is about 100 to 150 ppm though that is not a requirement forthis process. In that case the re-circulating water during evaporationmay need filtration to maintain solids content during evaporation. Inthis case the color removal process will make the neutralization andseparation of the salts easier during brine neutralization.

The brine 121 is taken for neutralization. In the neutralization processthere is no jelly formation. There is also significant reduction inchemical consumption because most of the acid-consuming ions do notexist in the brine. The brine 122 at this stage is ready for disposalafter separating any solids during neutralization. The brine can also bedisposed of without removal of solids after neutralization 122 asrequired by local regulations. Brine 121 can also be used for mixingwith solids after color removal step and/or silica removal step if thepreference is not to send any solids for disposal in land fill due tolocal preference or constraints.

Alternatively the brine 121 can be further concentrated in crystallizer124 by recovering 75-80% of the balance water after evaporation. Thedistillate 125 may be recycled for beneficial use. The slurry 126 leftin the crystallizer is either sent for disposal after dewatering 127 orbrine neutralization 122 or alternatively converted into solids withsuper absorbents 123 and sent for disposal. The resultant solid can alsobe incinerated if required. The color removal step increases waterabsorption capacity of super absorbents and makes such absorptionfaster. For analysis of color, especially for highly concentratedsolutions after evaporation, a dilution method was used and the readingswere adjusted based on factors of dilution.

Embodiments of the invention may include one or more deaeration steps.For example, deaeration may be used after color removal and solidseparation and before pH is increased. It may also be used after asecond-stage removal of silica. Typically deaeration occurs after sludgeremoval.

The applicants stress that although the utility of color removal in thesteam flood process can not be underestimated, the process reportedherein may be equally useful when preceding a reverse osmosis plant,with or without silica removal. It may also be useful without silicaremoval, as a pretreatment followed by a filtration process, likeultrafiltration and softening, to remove any residual hardness.

Example 1

A color removal experiment was performed as per the initial step of theembodiment shown in FIG. 2. To 3000 mL of produced water was added 5.8mL of a 10% sulfuric acid solution to reduce its pH to 4.2. This wasfollowed by addition of 100 ppm color removal chemicals, Wexfloc-9 (30mL of 1% Wexfloc-9 solution). A mixing retention time of 15 minute wasgiven and then 0.5 ppm of flocculent (1.5 mL of 0.1% AT-7594 solution)was added to the water. The heavy sludge settled within 5 minutes buttreated water was decanted after 30 minute for analysis. The results areshown in Table 1.

The pH & conductivity were checked by laboratory instruments. The colorwas analyzed as per APHA Platinum-Cobalt standard method through a HACHDR/2010 spectrophotometer. The silica was analyzed by silicomolybdatemethod through a HACH DR/2010 spectrophotometer. TSS was analyzedthrough filtration and gravimetric method as per APHA total suspendedsolids method. TOC was analyzed through TOC Analyzer (Model: TOC-L CPH,Shimadzo corporation).

TABLE 1 Color Removal Process Raw Produced Treated Parameters waterwater Reduction pH 8.05 4.22 Conductivity, μS/cm 3130 3300 Color, PtCounit 4260 360 91.55% TOC, ppm 389.6 145 62.78% Silica as SiO2, ppm 240160  33.3% TSS, ppm 140 14   90%

It was evident that around 90% of color removal and 60% of organicsremoval were easily achievable by this process. The process furtherreduced around 33% of silica from the water.

Example 2

In another experiment produced water was treated as shown in FIG. 3. Theproduced water was first treated with a color removal process asdescribed in Example 1. In the decanted treated water, magnesium oxidewas added with constant agitation. The water was continuously mixed for15 minutes, and then its pH was increased to 10 by the addition of lime.Finally the pH was further increased to 11 by the addition of sodiumhydroxide. Then 2.5 ppm of coagulant AT 7260 was added to the water andmixed for another 2 minutes. The sludge settled within 15 minutes ofretention time in a solids separator, and supernatant treated water wasdecanted. A portion of treated water was analyzed and results are shownin Table 2.

The pH & conductivity were checked by laboratory instruments. The colorwas analyzed as per APHA Platinum-Cobalt standard method through a HACHDR/2010 spectrophotometer. The silica was analyzed by a silicomolybdatemethod through a HACH DR/2010 spectrophotometer. TSS was analyzedthrough filtration & gravimetric method as per the APHA total suspendedsolids method. Hardness was checked by EDTA Titrimetric method. TOC wasanalyzed through a TOC Analyzer (Model: TOC-L CPH, Shimadzocorporation).

TABLE 2 Treated water Raw Produced (after Silica Removal parameterswater Removal Step) Efficiency (%) pH 8.05 11.21 Conductivity, μS/cm3130 3670 Color, PtCo unit 4260 650 84.74% TOC, ppm 389.6 122 68.68%Silica as SiO2, ppm 240 8 96.67% TSS, ppm 140 6 95.71% Hardness asCaCO3, 35 20 ppm

The results clearly indicated that silica in treated water by thisprocess was below 10 ppm level, around 96.7% reduction was achieved, andcolor reduction was around 85% and TOC reduction was around 68.7%. Itwas seen that color values showed a higher reading at alkaline pH.

A further set of tests was conducted on decanted treated water ofExample 2. Around 2600 mL of treated water was passed through anevaporation set up and 97% of distillate recovered from the treatedwater. During evaporation water pH was maintained around 10-10.5 bysodium hydroxide. When the brine volume reduced to 75 mL, a portion ofit was analyzed for color and silica content and their results are shownin Table 3. The pH and conductivity were checked by laboratoryinstruments. The color was analyzed as per the APHA Platinum-Cobaltstandard method, and silica was analyzed by a silicomolybdate methodthrough a HACH DR/2010 spectrophotometer.

TABLE 3 Treated water Raw Produced (after silica Brine parameters waterremoval step) water Water volume 3000 mL 2600 mL 75 mL pH 8.05 11.2110.68 Conductivity, μS/cm 3130 3670 153800 Color, PtCo unit 4260 6503500 Silica as SiO2, ppm 240 8 300

During evaporation, no foaming was observed, which otherwise is a commonobservation in the conventional process. No scaling was observed onevaporator surfaces. The brine was clear, with a significantly loweramount of precipitates. There was no tarry appearance, because colorunits in the brine were still lower than the color units in the originalwater.

A further set of tests was conducted on brine water. The brine water (75mL) was further concentrated up to 19 mL and recovered 75% of waterafter evaporation and then the concentrated slurry was neutralized to pH9.0 by acid. No jelly formation occurred on concentrated slurry, whichwas neutralized with 1.8 mL of 5N hydrochloric acid. The slurry wasanalyzed, and results are shown in Table 4. The pH and conductivity werechecked by laboratory instruments. The color was analyzed as per theAPHA Platinum-Cobalt standard method, and silica was analyzed bysilicomolybdate method through HACH DR/2010 spectrophotometer.

TABLE 4 parameters Concentrated slurry water Slurry Volume 19 mL Sludgevolume in slurry 25% Water volume in slurry 75% pH value 9.0Conductivity, μS/cm 352500 Color, PtCo unit 22750 Silica as SiO2, ppm975

The absorbance of neutralized slurry was checked on super absorbent. Theneutralized slurry, weight around 20 gm, was absorbed on 1.25 gm ofsuperabsorbent. The whole slurry was absorbed on superabsorbent in twohour time period.

Comparative Example 1

In this comparative experiment the 3000 mL of produced water was treatedwithout color removal as shown in FIG. 4. The produced water was firsttreated by magnesium oxide, lime, sodium hydroxide and coagulantAT-7260. The produced water and dosing chemicals quantity were similaras used in Example 2. It was observed that the sludge settling rate wasslow, and it settled in 2 hours of retention time. The supernatanttreated water was decanted and a portion of it was analyzed for silica,color, pH and other parameters. The results are shown in Table 5.

The remaining 2700 mL of treated water was passed through evaporator forevaporation. In a similar fashion, 97% of distillate was recovered fromthe evaporator, keeping the water pH around 10-10.5 in evaporator. Thebrine water volume was 80 mL. Results are shown in Table 5. The pH andconductivity were checked by laboratory instruments. The color wasanalyzed as per the APHA Platinum-Cobalt standard method through a HACHDR/2010 spectrophotometer. The silica was analyzed by thesilicomolybdate method through a HACH DR/2010 spectrophotometer. TOC wasanalyzed through a TOC Analyzer (Model: TOC-L CPH, Shimadzocorporation).

TABLE 5 Treated water Produced (after silica Brine parameters waterremoval step) water Water volume 3000 mL 2700 mL 80 mL pH 8.05 11.3410.46 Conductivity, μS/cm 3130 3340 154400 Color, PtCo unit 4260 161052100 Silica as SiO2, ppm 240 31 1460 TOC, ppm 389.6 225 —

In this comparative experiment, only 87% of silica, 62% of color andonly 42% of TOC reduction occurred in silica removal process. Duringevaporation foaming and scaling were observed in evaporator. The colorof brine water of this comparative experiment was almost 15 timesgreater than the brine color of a process using an embodiment of theinvention (Example 2) and silica content was also 5 times larger thanExample 2's brine silica.

The results of this comparative example clearly indicated thesignificance of a color removal process as reported herein.

The brine water was further concentrated and distillate recovery wasattempted. Around 72% of water was recovered from brine water duringconcentration. The brine water became a dark, tar-like slurry. A portionof slurry was analyzed and results are shown in Table 6. The pH andconductivity were checked by laboratory instruments. The color wasanalyzed as per the APHA Platinum-Cobalt standard method, and silica wasanalyzed by a silicomolybdate method through a HACH DR/2010spectrophotometer. The slurry was neutralized by acid.

TABLE 6 Concentrated slurry water of parameters comparative example-1Slurry Volume 22 mL Sludge volume in slurry 75% Water volume in slurry25% pH value 9.0 Conductivity, μS/cm 313000 Color, PtCo unit 115000Silica as SiO2, ppm 1890

The following disadvantages were observed during evaporation of treatedwater and neutralization of concentrated slurry in the ComparativeExample 1 experiment.

-   -   Foaming and scaling on evaporator during evaporation due to        excess color and silica.    -   More acid consumption as compare to the inventive process.    -   Foaming and jelly formation during neutralization due to excess        silica & other inorganic chemicals.    -   Dark, tar-like appearance of slurry.    -   Less water recovery as compare to the inventive process.

Comparative Example 2

In this comparative experiment, 2900 mL of produced water was treated asin a conventional process like that shown in FIG. 5. The color as wellas silica removal process was not performed in this experiment. Theproduced water was directly processed for evaporation. The pH of theproduced water was adjusted to 10-10.5 by sodium hydroxide and then theproduced water passed through an evaporator. Around 97% of distillatewas recovered from the evaporator. Significant foaming and heavy scalingwere observed during evaporation. A portion of brine water was analyzed,and results are shown in Table 7. The pH and conductivity were checkedby laboratory instruments. The color was analyzed as per the APHAPlatinum-Cobalt standard method, and silica was analyzed by asilicomolybdate method through an HACH DR/2010 spectrophotometer. TOCwas analyzed through a TOC Analyzer (Model: TOC-L CPH, Shimadzocorporation).

TABLE 7 parameters Produced water Brine water Water volume 2900 mL 85 mLpH 8.05 10.30 Conductivity, μS/cm 3130 141000 Color, PtCo unit 4260118000 Silica as SiO2, ppm 240 5120

In a further set of tests, an attempt was made to concentrate the brinewater further, but only 59% of distillate recovery would be possiblefrom brine water. It became a dark colored tar-like slurry as its colorwere observed as 267000 PtCo unit. This slurry contained a significantlylower amount of water and was very difficult to neutralize by acid. Aportion of the water was analyzed, and results are shown in Table 8. ThepH and conductivity were checked by laboratory instruments. The colorwas analyzed as per the APHA Platinum-Cobalt standard method through aHACH DR/2010 spectrophotometer.

TABLE 8 Concentrated slurry water of parameters comparative example-1Slurry Volume 35 mL Sludge volume in slurry 90% Water volume in slurry10% pH value 9.0 Conductivity, μS/cm 298000 Color, PtCo unit 267000

In this Comparative Example 2, the following disadvantages were observedduring treatment:

-   -   Significant foaming and severe scaling on evaporator during        evaporation due to excess of color and silica in brine water.    -   More acid consumption for neutralization of slurry. Foaming and        jelly formation during neutralization due to excess color and        silica contents.    -   Dark, tar-like appearance of slurry. Difficult to remove the        scaling and tar-like solids from vessel.

The advantages of various embodiments of the invention werewell-illustrated by a tabular comparison of the chemical consumptionnecessary to treat 3000 mL of produced water in the experimentsdiscussed above. These numbers clearly showed the reduction in chemicalconsumption in the pretreatment process and also acid consumption beforethe final disposal process.

TABLE 9 Comparative Comparative Example-1 Example-2 Example-1 Example-2Chemicals Description Experiment Experiment Experiment ExperimentSulfuric Acid (10% Conc.) for color removal 5.8 mL 5.8 mL N/A N/Aprocess Magnesium Oxide (solid, 97% pure) for silica N/A 1.5 gm 1.5 gmN/A removal process Lime (solid, 90% pure) for silica removal N/A 0.2 gm0.341 gm N/A process Sodium Hydroxide (10% Conc.) for Silica about 9 mL8.3 mL 11.5 mL 12.9 mL removal & evaporation process HCl (5N Conc.) forBrine Neutralization N/A 1.8 mL 2.5 mL 3.5 mL process up to 9.0 pH ColorRemoval Polymer Wexfloc-9 (1% Conc.) 30 mL 30 mL N/A N/A Polymer AT-7594(0.1% conc.) 1.5 mL 8.5 mL 7.5 mL N/A N/A—Not applicable

We claim:
 1. A process for purification of water, comprising, in order:collecting produced water including an oil/water mixture from an oilwell; separating and recovering oil from the produced water; decreasingthe pH of the produced water by addition of acid; removing color,organics, and silica from the produced water; adding a flocculent,including, optionally, a coagulant and a polyelectrolyte, to theproduced water to aggregate solids in the produced water; removingsolids from the produced water, thereby producing sludge; conditioningand disposing of the sludge; mixing the produced water with one or moreof lime, magnesium oxide, magnesium hydroxide, calcium oxide, and sodiumhydroxide, thereby precipitating silica from the produced water;removing precipitated silica as a second sludge; conditioning anddisposing of the second sludge; sending the produced water to anevaporator, producing a distillate stream and a brine stream; sendingthe distillate stream to a boiler for evaporation into steam; andsending a portion of the brine stream to a crystallizer for removal ofadditional distillate and production of a slurry to achieve zero liquiddischarge without formation of a tarry mass.
 2. The process of claim 1,wherein between 60-80% of organics, 80-95% of color, and more than25-50% of silica are removed from the produced water in the colorremoving step.
 3. The process of claim 1, wherein said brine stream isconverted to a solid through absorption with super absorbent polymerafter production of the slurry.
 4. The process of claim 1, wherein saidslurry is neutralized to a pH between 8.5 and 9 with the addition ofacid and without formation of a silica jelly.
 5. The process of claim 1,further comprising de-aerating the produced water after at least one ofthe step of removing solids from the produced water, thereby producingsludge, and the step of removing precipitated silica as a second sludge.6. The process of claim 1, wherein the step of decreasing the pH of theproduced water by addition of acid decreases the pH of the producedwater to between 3 to
 5. 7. The process of claim 6, wherein said acidaddition lowers the pH of the produced water to between 4 to 4.5.
 8. Theprocess of claim 1, wherein the step of mixing the produced water withone or more of lime, magnesium hydroxide, calcium oxide, and sodiumhydroxide adjusts the pH of the produced water to between 10-11.5. 9.The process of claim 1, wherein sending a portion of the brine stream toa crystallizer for removal of additional distillate and production of aslurry achieves zero liquid discharge.
 10. A process for purification ofwater, comprising, in order: collecting produced water including anoil/water mixture from an oil well; separating and recovering oil fromthe produced water; decreasing the pH of the produced water by additionof acid; removing color, organics, and silica from the produced water;adding a flocculent, including, optionally, a coagulant and apolyelectrolyte, to the produced water to aggregate solids in theproduced water; removing solids from the produced water, therebyproducing sludge; conditioning and disposing of the sludge; addingsodium hydroxide to the produced water to increase pH of the producedwater; sending the produced water to an evaporator, producing adistillate stream and a brine stream; sending the distillate stream to aboiler for use as steam in injection to an oil well; and sending aportion of the brine stream to a crystallizer for removal of additionaldistillate and production of a slurry to achieve zero liquid dischargewithout tar formation.